1. Field of the Invention
The present invention pertains to a method of recovering lithium chloride. More specifically, the present invention pertains to a method of economically and efficiently recovering a lithium chloride which is used as a polymerization agent in the production of polyarylene sulfides.
2. Description of the Prior Art
Polyarylene sulfides having a structure in which phenylene groups are chained by way of thioether bondings are generally used in various fields, such engineering plastics, because of their excellent mechanical properties, chemical resistance, heat resistance, etc.
These polyarylene sulfides are produced by reacting a dihalogenated aromatic compound and an alkali metal sulfide in a polar solvent.
If a lithium salt such as lithium chloride is used as a polymerization agent, a white polyarylene sulfide with less salt content can be obtained at high degrees of polymerization, and a polyarylene sulfide of optional molecular weight can be obtained.
However, since lithium chloride is expensive, it is necessary to recover and reuse the same in order to lower the cost of producing polyarylene sulfide, and it has also been desired to use a recovery method therefore, which is simple.
Although lithium may function as polymerization agents to the same extent as lithium chloride, it is difficult to recover them.
Heretofore, as the method of recovering the lithium salt from a reaction mixture of polyarylene sulfide, it has been known, to remove the solvent from the reaction mixture, wash the solid obtained therefrom with an aqueous solution of an acid such as dilute hydrochloric acid and blowing carbon dioxide into the washing solution or adding a carbonate such as sodium carbonate to deposit lithium carbonate, and filtering to recover lithium carbonate. Alternatively, hydrochloric acid may be added to the lithium carbonate, followed by evaporation to dry the resultant aqueous solution to recover lithium chloride (refer to Japanese Patent Laid-Open No. 202222/1984).
However, this method is difficult in that: (1) the recovery procedures are complicated and require complicated recovery facilities and a great amount of carbonate, etc. for depositing lithium carbonate; (2) direct and simple recovery of lithium chloride, if it is used as the lithium salt for the polymerization agent, is substantially impossible, and (3) unreacted oligomers or impurities are incorporated into the resultant polyarylene sulfide to reduce the quality thereof.